1. Field of the Invention
This invention relates to a process for the preparation of Ni-based superalloys and articles thereof.
2. Description of the Related Art
The performance of gas turbine engines is limited by the high temperature capability of turbine blades and nozzle guide vanes (NGV). These blades and vane parts operate in extremely hostile environment of high temperatures, high stresses, oxidation and hot-corrosion as high pressure hot combustion gas-air containing highly corrosive ingredients is directed against NGV parts at high velocity in such engines and the gas then imparts its theremo-kinetic energy to the turbine rotor blades. Ni-base single crystal superalloys have so far surpassed other materials in performance, in such environments. Among these single crystal superalloys the Re-bearing alloy CMSX-4 was performing earlier with highest turbine entry temperature (TEL) in a variety of aero-engines. Further improvements have been reported where an alloy CMSX-10 has been developed incorporating about 5 to 7 wt % if Re as compared to 3 wt. % Re in CMSX-4. This alloy has exhibited highest ever creep resistance among the prior art materials upto the temperature of 1150 C. together with reasonable oxidation and hot-corrosion resistance. This was owing to its higher total refractory element (W+Re+Mo+Ta) content cf 20.7 wt % together with higher Re content as compared to 16 and 3 wt. % respectively in CMSX-4 alloy. Sluggish diffusion of heavy refractory elements is known to enhance high temperature creep resistance in the gas turbine operating load and temperature conditions. Further rise in total refractory element content with the alloying combination of CMSX-10 however has not been possible because of phase instability that deteriorates creep resistance considerably. There is, therefore a need for the preparation of such Ni-based single crystal superalloys wherein total refractory content can be favourably increased beyond that of CMSX-10 alloy in order to achieve superior creep resistance along with enhancement in other useful characteristics like single crystal processability.
The turbine blades and vanes being essential elements of the engine, even a minor improvement in material characteristics has significant effect on the performance economics and life of the engine. More 10.degree. C. improvement in metal temperature capability can roughly double the component life and just 1.degree. C. advance in metal temperature capability amounts to 4-5.degree. C. increase in TET of hot combustion gas through blade cooling principle. Higher TET leads to better thermal efficiency, greater thrust and reduction in fuel consumption.
Another disadvantage of the processes in the known art is the requirement of ceramic cores for making hollow components like gas turbine blades and vanes, which leads to higher rejection of components as compared to the process proposed in the present invention.
Further disadvantage of the process known in the art is the molten alloy and ceramic core reaction that leads to appreciable loss of key alloying elements which is turn leads to performance deterioration in the materials.
The primary object cf the present invention is to propose a Ni-based superalloy and articles made therefrom having higher refractory element content beyond that of CMSX-10alloy and heat treated to achieve superior creep resistance, adequate resistance to oxidation, hot-corrosion and deleterious phase formation for application such a gas turbine blades and vanes.
An object of the present invention is to propose a Ni-based superalloy with excellent castability of advanced gas turbine blades and vanes in single crystal from having thin walled aero-foils, shrouded segments and intricate cooling channels.
A further object of the present invention is to propose a process for preparation of hollow single crystal superalloy components having complex cooling channels without employing ceramic cores thereby improving the yield of quality components.
Yet a further object of the present invention is to propose a Ni-based superalloy and articles made therefrom that can be heat treated without causing any recrystallization to achieve improved balance of critical mechanical properties for advanced gas turbine engine application.
A still further object of the present invention is to propose a Ni-based superalloy and articles made therefrom that can be conveniently brazed and given protective coating by existing manufacturing techniques during heat treatment.
These and other objects and advantages will be more clearly understood from the following detailed description, drawings and specific examples which are intended to be typical of, rather than in way limiting on the scope of the present invention.